Added the support of `--enable-valgrind-regtest=ignore-errors`.

* bootstrap: Added the support of `--enable-valgrind-regtest=ignore-errors`.
* jamroot: Added the support of `--enable-valgrind-regtest=ignore-errors`.
* valgrind/jamfile: Told build script to ignore errors in `make regtest` if
                    `VALGRIND_REGTEST` is equal to `ignore-errors`.